The present invention relates to a radiation curable coating composition comprising an aqueous dispersion of silica/vinyl-functional silanol dispersion in combination with one or more multifunctional acrylate or methacrylate monomers. The present invention also relates to an abrasion-resistant synthetic film comprising a synthetic film and the cured coating composition. Finally, the present invention also relates to processes for coating the radiation curable coating composition onto synthetic films, preferably onto polyester film.
Polyester films have recently found expanded applications as transparent and translucent layers applied to solid substrates and especially to transparent substrates. Thus, such films may be applied to windows and viewing or lighting media to control the transparency thereof. Additionally, polyester films have found many varied applications as independent substrates including uses in the optical arts for display purposes and in conjunction with electronic equipment having visual or optical screens and/or overlays. Also, recently developed membrane touch switches may contain an outer layer of polyester film. An extremely important application is the automotive windshield (laminated safety glass) market.
While polyester films have many desirable physical properties for such applications, these films unfortunately lack good abrasion resistance. Prior artisans have suggested coating such films with a wide variety of scratch-resistant coatings, including coatings containing one or more polyorganosiloxane compounds.
Many of the previous proposals have involved an aqueous dispersion of silica and at least one polyorganosiloxane which is applied to a synthetic substrate, such as polyester, and then thermally cured. For example, R. Ubersax, "Coated Polymeric Substrates," U.S. Pat. No. 4,177,315 discloses an abrasion resistant coating composition comprising from about 5 to 50 weight percent solids comprising from about 10 to 70 weight percent silica and about 90 to 30 weight percent of a partially polymerized organic silanol of the general formula RSi(OH).sub.3, wherein R is selected from methyl and up to about 40% of a radical selected from the group consisting of vinyl, phenyl, gamma-glycidoxypropyl, and gamma-methacryloxypropyl, and about from 95 to 50 weight percent solvent, the solvent comprising about from 10 to 90 weight percent water and about from 90 to 10 weight percent lower aliphatic alcohol, the coating composition having a pH of greater than about 6.0 and less than about 8.0. The above described coating composition is applied to the polyester substrate and subsequently cured at a temperature of from 20.degree. to 150.degree. C., preferably from 100.degree. to 150.degree. C. Curing catalysts such as alkali-metal salts of carboxylic acids and quaternary ammonium carboxylates can be included in the coating composition. Another example is D. Olson et al, "Abrasion Resistant Silicone Coated Polycarbonate Article," U.S. Pat. No. 4,239,798 which discloses a polycarbonate substrate primed with a thermoplastic acrylic polymer which is subsequently coated with a thermoset, silica-filled, organopolysiloxane top coat. This top coat is the condensation product of a silanol of the formula RSi(OH).sub.3 in which R is selected from the group consisting of alkyl radicals of 1 to 3 carbon atoms, the vinyl radical, the 3,3,3-trifluoropropyl radical, the gamma-glycidoxypropyl radical and the gammamethacryloxypropyl radical, at least 70 weight percent of the silanol being CH.sub.3 Si(OH).sub.3.
A number of patents teach the use of an abrasion resistant coating comprising a dispersion of colloidal silica in an aliphatic alcohol/water solution of the partial condensate of an alkyltrialkyoxysilane, which must be primarily methyl trimethoxysilane. These patents typically improve upon the abrasion resistance of this coating by incorporation of an additional component. A representative example is U.S. Pat. No. 4,277,287 which teaches the inclusion of a small amount of a polysiloxane polyether compolymer. Other patents which may be included within this group are U.S. Pat. Nos. 4,474,857; 4,436,851; 4,413,088; 4,373,061; 4,368,241; 4,368,236; 4,368,235; 4,367,262; 4,298,655; and 3,986,997.
There are a few patents which teach the use of radiation curable polysiloxane-based abrasion-resistant coatings. For example, V. Cross, "Polyester Film Having Abrasion Resistant Radiation Curable Silicone Coating," U.S. Pat. No. 4,310,600 teaches a coating composition comprising a silica/silanol composition being derived from a composition comprising a dispersion of colloidal silica in a lower aliphatic alcohol-water solution of the partial condensate of a silanol of the formula RSi(OH).sub.3 in which R is selected from the group consisting of unsubstituted and inertly substituted alkyl radicals having one to three carbon atoms and unsubstituted and inertly substituted phenyl radicals, at least about 70 weight percent of said silanol being of the above formula wherein R is CH.sub.3, said silica/silanol composition containing about 10 to about 70 weight percent of said colloidal silica and about 30 to about 90 weight percent of said partial condensate, based on the total solids weight; and one or more multifunctional acrylate monomers and/or oligomers; and optionally one or more photoinitiators.
Similarly, R. Chung, "Abrasion Resistant Ultraviolent Light Curable Hard Coating Compositions," U.S. Pat. No. 4,348,462 discloses a radiation curable coating composition which comprises (a) colloidal silica, (b) acryloxy or glycidoxy functional silanes, (c) non-silyl acrylates and (d) catalytic amounts of UV sensitive cationic and radical type photoinitiators. Other patents which disclose UV curable polyorganosiloxane coating compositions include U.S. Pat. Nos. 4,595,471 and 4,571,349.